From New Scientist 22 February 2014, page 14
The first cases have now been found in which the smoking of cannabis has caused deaths, with no other agent involved.
Recent post mortems of people who died after heavy cannabis use has shown three people whose deaths were unrelated to anything other than cannabis overdose. The metabolic cause of those deaths is yet to be determined, but it is likely that more such deaths will be identified in the future.
There are many apologists for cannabis, who will adamently claim that cannabis is harmless. Well, it appears to be otherwise.